Increase in recording capacity per unit area, i.e., areal recording density, of a magnetic disk (hereinafter simply called a disk) is required for a hard disk drive. The areal recording density is a product of linear density and track density. The linear density is the recording capacity per unit length along the track direction and is represented by, for example, the number of recording bits per inch, BPI. The track density is the recording capacity per radial unit length and is represented by, for example, the number of tracks per inch, TPI.
Servo data for positioning of a magnetic head (hereinafter simply called a head) at a target position, i.e., a target track on the disk is written on the disk. The servo data is written in servo areas discretely arranged with fixed intervals along a track direction.
For positioning of the head at the target track with high accuracy, based on the servo data, the shape of a center line of the track on which the servo data is written should desirably be a perfect circle. In general, however, the shape of the center line in the track is distorted and is not a perfect circle. The reason is mainly the accuracy in positioning at the time of writing the servo data. The positioning accuracy depends on deflection of the rotation axis of a spindle motor which rotates the disk, the tilt (flutter) component of the disk, deflection caused by airflow disturbance at the rotation, etc. If the shape of the center line in the track is distorted, the servo data includes an error caused by the distortion. Since the error is synchronous with the rotation of the disk, the error is called the repeatable runout (RRO). For this reason, after writing the servo data, RRO correction data for correction of the RRO is written subsequently with the servo data. At the positioning of the head, the servo data is corrected based on the RRO correction data and the head is positioned at the target track in accordance with the corrected servo data.
The servo areas in one track need to be increased in order to increase the positioning accuracy but, as the servo areas are increased, the data areas are reduced, the BPI becomes smaller and the linear density cannot be increased.
The data is not written at one time but is often overwritten on the same portion at several times. In this case, if the track width is small, i.e., if the TPI is great, the positioning accuracy at the writing becomes worsened and, if the head is displaced from the center of track, the data of an adjacent track may be erased by overwriting. The TPI is set at a small value including margin to some extent.